joining multi objects in blender to make one object
It sounds like you objects may be scaled or have non applied transformations

Joining with Ctrl + J is done in object mode, selecting all objects you want to join and selecting the "receiving object" last.

Try applying all transforms to the objects before joining with Ctrl + A > Apply Rotation, Translation and Scale.

After that also try converting all objects to mesh with Alt + C > Mesh to apply all modifiers and deformations they may have. Only after that can you safely try to join.

Otherwise post your blendfile so we can look at what might be wrong.